1. Field of the Invention
The present invention relates to a control device for a primary braking system, more particularly, to a primary retarder in a vehicle, as well as to a method to control a primary braking system.
2. Description of the Related Art
Primary braking systems generally refer to braking systems which are positioned in front of the clutch and the transmission. (Ref. Kfz-Anzeiger. 44th year, 1991, report: "Safely Downhill", page 30 and Lastauto Omnibus April 1991, report "Against the Current", page 30). Secondary braking systems, on the other hand, are mounted directly on the transmission or in the drivetrain downstream of the transmission, and act on the rear axles of the vehicle.
Next to primary retarders, there are the following additional primary braking systems that have become well-known in the industry: Guillotine-type exhaust brake, bleeder brake compression release brake.
The guillotine-type exhaust brake relies on the increased backpressure to generate the braking torque. During braking, the exhaust piping is nearly fully-blocked in order to impede the outflow of the cylinder charge during the exhaust stroke, thus elevating the braking power of the engine.
The bleeder brake (Ref. Kfz-Anzeiger. 44th year, 1991, report: "Safely Downhill", pages 10-13) is a type of brake which utilizes an additional valve, a so-called decompression valve which is integrated into the cylinder head. When activated during the third stroke of the cycle, it substantially lowers the expansion pressure acting on the piston and, thus, lowers the acceleration of the piston. Therefore, a difference develops between the compression and expansion work, which can be used to increase the braking power of the engine. In particular, such a braking system can be modulated by controlling the throttle positions.
In a further development of the bleeder brake, the decompression valve is not kept open during the entire working cycle, but only during a short time span (ref MTZ Motortechnische Zeitschrift 56 (1995) 7/8 pages 418-423; "The New Decompression Release Brake (DVB) from Mercedes Benz").
The control of such an engine brake can be performed by a proportional control valve, serving the function of a pressure control valve with the capability of affecting the opening characteristics of the decompression valve.
For further details, the above indicated article of the MTZ Motortechnische Zeitschrift 56 (1995) 7/8 page 422 is recommended as reference.
The compression release brake is, just like the bleeder brake, an engine brake. In the case of the compression release brake, a valve control device releases the compression, which leads to--as with the bleeder brake--a substantial increase in engine brake power (ref Kfz-Anzeiger, 47th year, January 1994, report: "Elegantly Packaged--Test Report Volvo FH", pages 10-12).
While the control device and the control method, in accordance to the present invention can be, as indicated above, applied to any primary brake systems, primary retarders also provide a unique application for this invention. It is, unlike the secondary retarder which is normally mounted between the transmissions and the propeller shaft, acting directly on the engine, as already described above. A primary retarder, which is permanently connected to the engine, is known from German Patent Document DE 44 08 349, incorporated herein by reference.
There are two principal operating modes available for the retarder during braking operations: "Braking mode" or "V-constant". The basic function "braking mode" is characterized by a fixed brake setting of the retarder. This can be achieved by use of a hand brake lever or a foot pedal, which, based on a fixed correlation, achieves a certain braking torque, which can range from a minimum braking torque M.sub.min to a maximum braking torque M.sub.max.
While operating the brake at "V-constant", the retarder can be, from a controls standpoint, coupled with the cruise control. The retarder braking action is achieved by adjusting the braking torque, so that a constant speed can be achieved, i.e., during downhill operation.
Control devices or methods for the control of a retarder, with respect to its braking torque, have been published in German Patent Document DE 43 41 213, which is incorporated herein by reference.
In order to avoid a conflicting operation between the retarder and the engine, i.e., at elevated speeds, German Patent Document DE 43 41 213 proposes to establish a relative priority of the individual systems for specific applications. For example, it is proposed to always yield the priority to the retarder in case the throttle and retarder are activated simultaneously. If the "constant velocity" function of the retarder is activated, the application suggests moving the throttle lever of the engine to the idle position.
Due to the direct connection, the braking torque of a primary braking system always correlates with the engine speed. The braking torque of a primary braking system which is mounted on the axle or drive wheels increases with the overall gear ratio of the transmission. This causes the braking moment acting upon the braking wheels to be--under some circumstances--exceeded while braking in low gear during downhill operation, leading to unacceptable wheel slips.